The superheterodyne is the most commonly used radio receiver system. As shown in FIG. 1, it can be seen that two local oscillators are required for receiving and a third LO signal is needed for transmitting in a transceiver system. The first LO will probably be generated by a VCO controlled by a synthesiser whilst the TX LO may be generated using a second frequency synthesiser or be produced by mixing the first and second LO signals. An RF filter is required to block unwanted radio signals and remove image frequencies which would otherwise be converted by the first mixer to an inband IF frequency. A second high frequency filter is required to remove unwanted side tones generated by the nonlinear action of the first mixer.
Direct conversion can be obtained by a homodyne system as shown in FIG. 2. This system is simplified by the need for only one LO signal which may also be directly used for tansmitting in a transceiver. Filtering is also easier to perform since the RF filter has only to provide a blocking function and all post mixer filtering is provided by low pass baseband filters. The advantage of these base band filters is that they may be totally constructed within an integrated circuit. Automatic gain control will be required which can be provided by an RF AGC 1 stage or a combined action of AGC 1 and base band AGC 2 stages as shown.
The disadvantage with this direct conversion system is that the local oscillator may radiate a signal which when picked up at the front end will be converted down and generate a DC offset in the mixer outputs. Since this signal may be a multipath reflected signal when received, it will probably have a varying amplitude and phase and hence when converted down could produce varying offsets i.e. unwanted base band signals. This problem is reduced by minimising the generated LO signal level, however this can result in decreased gain and noise performance in the mixers.